# Help designing a remote controlled cupboard lock

#### Asheekay

##### New Member
Out of the 3 cupboards I use in my home, there's one that also holds items sometimes needed by others.

This cupboard, I would like to be able to unlock from afar, without using manual keys, of course.

After thinking and trying a few options including:

1- disassembling and scavenging a small drone quadcopter for its remote controlled part (and failing miserably at it)
2- using several models of remote electronic car lock gadgets (they turned out to have only a range of up to 10 meters max)

I finally came up with the idea of somehow controlling it with a cellphone. For one, cellphones have infinite range (provided service availability of course) and the usage is pretty easy (you just call or message, right?). Initial research revealed that I have two options for the project:

1- manual approach (which I like), which turns on the lock's dc motor if there's input on the cellphone's speakers
2- digital approach, using an arduino uno, which does the same thing, but also has option for operating it through a specific message. While providing far better control, his is also far more complex for me, hence not the preferred method

So now the question is, how can I turn on a 12v dc motor just by calling on my cell number? I have an old B&W nokia to spare for this job.

I have tried connecting the speaker cables directly to the motor, but that doesn't even rotate the shaft at all. I guess I will have to power the motor separately through a 12v battery and somehow trigger it from the cellphone's speaker cables. And this is where I need help. What I have in mind currently is this:

What I need help about is:

1- Is this circuit practically applicable?

2- Should I indeed use a transistor to switch on the motor as compared to a relay? I have checked that the cellphone speakers cables provide a maximum of 3v, 100 mA power. Will a relay turn on at all at this power?

3- And finally, go on and tell me that the transistor will fuse away due to excess current. Which resistor(s) do I need to use in this setup (or any that you suggest) to keep it going?

#### Nigel Goodwin

##### Super Moderator
No it's not practical - you can't just use a speaker output in that way.

As you've already considered, using a GSM modem and a processor (such as an Arduino) is probably the way to go.

Another option would be an ESP8266 or ESP32 running a webserver, on your local WiFi and access that externally via a web browser on your phone.

There are multiple examples of both methods available on line.

#### Asheekay

##### New Member
I try to stick to cellular data on my smartphone, so anything using WiFi would be complicated. While possible definitely, it will be much more costly than just unlocking the thing with just a ring.

Arduino would have been a great solution if not having two issues:
1- I don't know arduino programming (I know C, and arduino language is primarily C based, but I don't have the confidence / experience and I have zero knowledge of the arduino functions / operators / APIs)

2- It will be much more costly than just exploiting the speaker out signal

Any method to do that?

#### Nigel Goodwin

##### Super Moderator
2- It will be much more costly than just exploiting the speaker out signal

Any method to do that?
Of course, but it will be triggered by ANYONE ringing the phone, including text messages from your phone provider.

Some kind of signal detection system on the output, feeding a relay driver - essentially it's a sound activated switch.

However, WiFi wise, almost every basic example for the ESP8266 shows a webserver that produces a web page with two buttons ON and OFF - when you click on the buttons it turns the in-built LED ON and OFF. All you need to do is connect a relay driver to the I/O that feeds the LED, and put your own WiFi details in the source code. Job done - if you want to access it from anywhere in the world?, you simply need to port the access for it, so you can use it externally.

#### JonSea

##### Well-Known Member
In this case, you might be best to buy parts to create what you want.

A Sonoff 12VDC module will allow control from a smart phone app anywhere in the world if you have a WiFi network at home. You'd want the 12 volt version of this module, which has something they call"inching mode" - when you turn it on via your phone, it will activate a relay for a few seconds, then turn off again.

Use this relay to control an electronic lock module similar to the one pictured.

You'll need to supply 12 volt DC power to the inside of the cupboard to operate everything.

#### Asheekay

##### New Member
Of course, but it will be triggered by ANYONE ringing the phone, including text messages from your phone provider.
Oh, that is easily fixed with using call barring. Calls from all numbers would be blocked, excluding my own or some other authorised person.

Some kind of signal detection system on the output, feeding a relay driver - essentially it's a sound activated switch.
Is it possible to make it current/voltage driven switch? I can find ringtone detector custom circuits available on eBay but I want to do this with some amount of accomplishment as an electronics enthusiast.

How do we go around harvesting the power output to the speaker, amplify it to 12v, rectify it and feed it to the motor? This is what I am looking for, please.

A Sonoff 12VDC module will allow control from a smart phone app anywhere in the world if you have a WiFi network at home. You'd want the 12 volt version of this module, which has something they call"inching mode" - when you turn it on via your phone, it will activate a relay for a few seconds, then turn off again.
I was suggested this approach by a peer, but it uses WiFi, which is a hassle and additional expense to provide, plus SONOFF is quite expensive, too.

If you do not want WiFi for some reason, there are many Bluetooth locks for cabinets and the like e.g., https://www.smartlockssupplier.com/products/Smartphone-APP-Controlled-Bluetooth-Cabinet-Lock.html
That's completely out of the question, friend.

I am looking for solutions which enable operating the lock from my workplace. That's around 40 km from my home.

#### DrG

##### Active Member
That's completely out of the question, friend.

I am looking for solutions which enable operating the lock from my workplace. That's around 40 km from my home.
Yes, Bluetooth certainly would be out of the question...now that you have defined 'afar'

his cupboard, I would like to be able to unlock from afar, without using manual keys, of course.
You seem locked into your "ring" method - Good luck with it and let us know how it goes.

#### Asheekay

##### New Member
Actually yes. My mind is stuck with this approach as it enables me to actually "make" something as opposed to just ready-made available commercial circuits and just connect them to the motor.

It's sort of an obsession now, considering how almost everybody tells me how difficult it is to go this way.

#### DrG

##### Active Member
Let me see if I understand it correctly. If I want to get into the shared cupboard, I call you up when I am at the cupboard. You tell me to hang up and hold my phone up to the cupboard. You call my phone and the "ring" operates the lock - is that it?

Couldn't I just play the ringtone with my phone while at the cupboard and not bother calling you?

Edit: I don't know as that I am telling you that it is difficult, it is more that the performance may not be what you are expecting. If, of course, I am understanding it correctly.

#### Nigel Goodwin

##### Super Moderator
Actually yes. My mind is stuck with this approach as it enables me to actually "make" something as opposed to just ready-made available commercial circuits and just connect them to the motor.

It's sort of an obsession now, considering how almost everybody tells me how difficult it is to go this way.
We've never said it's 'difficult' it's just a poor way of doing it. The website I linked to gives you suitable circuits for doing it via audio, but you'd probably need to reduce the gain of the opamp as you're not using a microphone.

#### JonSea

##### Well-Known Member
I was suggested this approach by a peer, but it uses WiFi, which is a hassle and additional expense to provide, plus SONOFF is quite expensive, too.
A Sonoff module is quite expensive? Perhaps you didn't look at the link - it's less than US$7. If you don't have WiFi available, it won't work, but I think that's going to be cheap by the time you make your idea work. Whatever you do with the signal coming from the phone, you will have to have a separate power source to run the lock - there's not enough power coming from the headphone jack to make it work. #### Asheekay ##### New Member Let me see if I understand it correctly. If I want to get into the shared cupboard, I call you up when I am at the cupboard. You tell me to hang up and hold my phone up to the cupboard. You call my phone and the "ring" operates the lock - is that it? Couldn't I just play the ringtone with my phone while at the cupboard and not bother calling you? Oh no no. Not at all, senior man. It goes like this. I take my old defunct Nokia 1280 cellphone, remove the back cover and disconnect the cables going to the speaker. I use these speaker cables to the amplifier circuit and the output to the lock motor. This cellphone is kept inside the cupboard. If someone wants me to unlock the cupboard, they call me. I tell them to wait and then call my that old Nokia phone. The ringing signal feeds the amplifier and that turns the motor, hence unlocking the cupboard. The main issue here is, how do I amplify the ringing signal up from 3 volts 100 mA to 12 volts 100 mA? A Sonoff module is quite expensive? Perhaps you didn't look at the link - it's less than US$7.
Import taxes and then sales taxes ... It would arrive at my doorstep at around 24 USD.

#### Nigel Goodwin

##### Super Moderator
The main issue here is, how do I amplify the ringing signal up from 3 volts 100 mA to 12 volts 100 mA?
I've already told you, and posted a link to suitable circuits - the ringing level also isn't 3V 100mA anyway - those kind of figures don't apply to speaker outputs.

#### DrG

##### Active Member
Oh no no. Not at all, senior man.
Umm ok, I guess amplification is the only thing holding you back...now that you have that pesky "dedicating a phone to unlock a shared cupboard" sorted out.

Sorry I could not be more helpful, but it is time for my nap.

#### gophert

##### Well-Known Member
Oh no no. Not at all, senior man.
...
Import taxes and then sales taxes ... It would arrive at my doorstep at around 24 USD.

It's 0 Rs.

Here in Pakistan, we have some of the cheapest calling and messaging rates. You don't require any credit/service fee to receive calls and messages. And to make calls and messages, it's around 400 Rs ($3.8 approx) per month. And that is precisely why I so much prefer the call approach over arduino and WiFi. #### gophert ##### Well-Known Member Most Helpful Member It's 0 Rs. Here in Pakistan, we have some of the cheapest calling and messaging rates. You don't require any credit/service fee to receive calls and messages. And to make calls and messages, it's around 400 Rs ($3.8 approx) per month.

And that is precisely why I so much prefer the call approach over arduino and WiFi.
That explains a lot of things. Do you know other countries with similar business models (free incoming calls)?

#### Asheekay

##### New Member
No idea.

Here incoming is 100% free and outgoing is very cheap.

Don't ask about cellphone prices though ... they're around 150% of international prices